The encapsulation of cellulosic fibers in a polymeric matrix is known, having previously been disclosed for example in U.S. Pat. Nos. 3,836,412; 4,228,116; 4,248,743; 4,376,144; and 4,792,020.
U.S. Pat. No. 3,836,412 discloses composites of discontinuous organic fiber bonded in a vulcanized elastomer.
U.S. Pat. No. 4,228,116 discloses a process for producing remoldable panels by continuously plastifying and extruding an at least partially aggregated mixture comprising about 40 to about 60 weight percent cellulosic filler, and rolling the web of extruded material prior to its solidification.
U.S. Pat. No. 4,248,743 discloses dispersing wet wood pulp in a mixture of polyethylene, carbon black and ethylene vinyl acetate rubber in a Brabender mixer. The amount of fiber added ranged from 10.3 to 15.9 parts by weight fiber per 100 parts of the sum of polyethylene and rubber.
U.S. Pat. No. 4,376,144 dislcoses treating cellulose fibers with vinyl chloride polymer, a plasticizer and an isocyanate bonding agent to produce moldable or extrudable composites.
U.S. Pat. No. 4,791,020 discloses composites comprising wood fiber dispersed in and bonded to a mixture of polyethylene (or a copolymer of ethylene and propylene) and a compound containing at least one isocyanate group made by processing the mixture in an internal mixer or extruder, or on a roll mill.
In papers titled "Use of Eastern Hardwoods in Wood Fiber/Plastic Composites" and "Compounding Wood Fibers and Recycled High Density Polyethylene Using a Twin-Screw Extruder", Michigan State University (1988), researchers reported composites made by mixing up to 60 weight percent wood fibers into a high density polyethylene matrix in a twin-screw extruder. Use of a double planetary mixer equipped with a hot oil bath was not able to provide adequate melting of the HDPE and resulted in excessive charring and degradation of wood fiber. Mixing in a single-screw extruder only worked well for fiber levels up to about 10 percent by weight.
Notwithstanding the processes previously disclosed by others, there remains a need for a method of producing strong composite materials comprising cellulosic fibers dispersed in a polyolefinic matrix without requiring the use of vulcanizable elastomers, vinyl chloride polymer, or isocyanate bonding compounds.